1. Field of the Invention
This invention relates to gas turbine engines and more particularly to apparatus for supporting the fan blades of a turbofan engine.
2. Description of the Prior Art
The turbofan engine is the type of power plant most widely used on large aircraft today. In the turbofan engine as distinguished from a turbojet engine, a portion of the working medium gases is pumped axially through one or more compression stages and is exhausted directly to the atmosphere without passing through the core portion of the engine. The compression stages which exhaust directly to the atmosphere are called fan stages and are generally positioned at the forward end of the engine. The ratio of the air flowing through the fan stages to air flowing through the core portion of the engine is referred to as the bypass ratio. The bypass ratio may be a different value for each individual engine model according to the performance requirements for that power plant. In all turbofan engines, however, the fan stages make a substantial thrust contribution of between 30 and 75 percent to the total engine thrust at take-off with the actual contribution depending principally upon the bypass ratio.
The size and weight of the fan stages varies proportionately with the bypass ratio. In one typical engine, the JT9D turbofan engine manufactured by Pratt & Whitney Aircraft, a Division of United Aircraft Corporation, a single fan stage with large area flow path accommodates flow at a bypass ratio of approximately 5. Although the blades and the disk which comprise the fan stage of the JT9D engine are fabricated for titanium, the blades weigh collectively 450 pounds and the disk weights 470 pounds. Most turbofan engines in commercial service today produce proportionately high fan thrust at takeoff and have fan disks which correspond in size to the JT9D disk. Such a massive disk is required to distribute the disk hoop stress which is generated as the blades and disk are rotated at speeds in excess of 3200 revolutions per minute during operation of the engine. Additionally, the disk is sized to have adequate torsional and bending stiffness to resist deflection under the most critical vibratory excitations.
When compared to a turbojet engine operating at the same thrust level, one of the most attractive features of the turbofan engine is its relatively low level of noise generation. This low level of noise generation is made possible by reductions in the strength of the shear turbulence between the exhausted gases and the ambient medium. To increase the bypass ratio additional kinetic energy is removed from the core gas stream to drive the fan stages. Resultantly, the exhaust gas velocity of the core stream is diminished and a reduced level of shear turbulence obtains.
As engines having larger bypass ratios are designed and the size of the fan components is increased, significant performance increases can be achieved if the weight of the fan disk is reduced without sacrificing structural support for the blades mounted thereon.